Excavators are typically used in construction and reclamation or cleanup projects for the grading of land and dredging operations. Known excavators will have a clamshell bucket mounted to the end of a stretchable arm. The stretchable arm is normally defined by a two-member linkage. One of the linkages, called a boom, is pivotally mounted to a machine base of the excavator, and extends outwardly in an upward direction. The other linkage, called a stick arm, is pivotally mounted at one end to the outer end of the boom and extends downwardly from a boom pivot. Normally, the clamshell bucket is pivotally mounted to the outer end of the stick arm.
In operation, hydraulic cylinders of the excavator are typically used to move the boom, the stick, and the bucket independently under the control of an operator or a machine control system. The clamshell bucket itself is openable and closable by means of fluid pressure applied by a hydraulic cylinder. Another hydraulic cylinder may be used to rotate the machine base relative to a set of tracks. This permits a repositioning of the clamshell bucket for operations like cutting of the land and dumping to a desired location.
Most excavating projects involve creating surfaces that are substantially planar, either horizontal or sloped. Operating an excavator efficiently requires a skilled operator, especially when the excavator is being used to excavate sloped surfaces. Operator skill is especially critical because the couplings between the machine base, boom, stick arm, and bucket are pivots, and therefore extending or retracting any single hydraulic cylinder or actuator causes the digging edge of the bucket to move in an arc. The clamshell bucket will also usually have a rectangular-shaped footprint. For these reasons, excavating operations for the formation of sloped surfaces usually require approaching the surface to be excavated from suitable locations relative to the sloped surface, and precise use of the clamshell bucket.
However, even with the most skilled operators, use of conventional clamshell buckets has been found to result in an undesirable degree of bucket overlap when cutting a sloped surface. This is wasteful and not environmentally sensitive. Maintaining the optimal orientation of the bucket to the sloped surface with conventional clamshell buckets has also proven difficult, especially with the rectangular-shaped footprint associated with conventional clamshell buckets.
There is also a particular need for a clamshell bucket to meet the stringent requirements associated with environmental dredging. Such environmental dredging work includes the removal of polychlorinated biphenyl (PCB) contaminated sediment, transportation of sediment, and disposal of the sediment into an existing Confined Aquatic Disposal (CAD) cell. The bucket required position accuracy for such projects is +/− four (4) inches vertically and +/− six (6) inches horizontally. The work is required to be conducted in a two-pass approach. The first pass removes the material up to one (1) foot above the required design. The second pass removes the final one (1) foot of material to the required design. The dredging is also required to be conducted from the top-down (shallow to deep) in order to minimize residuals.
There is a continuing need for an excavator bucket that facilitates the cutting of a sloped surface, and minimizes bucket overlap when cutting the sloped surface. Desirably, the excavator bucket also permits the maintenance of a predetermined orientation of the bucket to the sloped surface and allows for compliance with stringent environmental dredging requirements.